Message details: group by send status, including viewed state

ts/util/mapUtil.ts

@@ -0,0 +1,26 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { reduce } from './iterables';
+
+/**
+ * Like Lodash's `groupBy`, but returns a `Map`.
+ */
+export const groupBy = <T, ResultT>(
+  iterable: Iterable<T>,
+  fn: (value: T) => ResultT
+): Map<ResultT, Array<T>> =>
+  reduce(
+    iterable,
+    (result: Map<ResultT, Array<T>>, value: T) => {
+      const key = fn(value);
+      const existingGroup = result.get(key);
+      if (existingGroup) {
+        existingGroup.push(value);
+      } else {
+        result.set(key, [value]);
+      }
+      return result;
+    },
+    new Map<ResultT, Array<T>>()
+  );